I don't read much fiction, but someone recently pointed out that several of Robert E. Howard's Conan stories are available for free from Project Gutenberg. I had previously read The Hour of the Dragon, the most-downloaded of the lot, and found it alright but not great. The seams of the serial structure in which it was originally published showed through pretty hard. The next most-read on Gutenberg was Red Nails.
I enjoyed Red Nails quite a bit (with the usual caveats about '30s pulp fiction), but more important, this is really good inspiration for old-school style dungeoncrawls. Without spoiling anything, there's a multi-level dungeon with competing factions, and those factions aren't monolithic, precisely in the way that dungeon factions are talked about in the OSR blogosphere. There are also a couple neat ideas for magic items and dungeon set dressing pieces. The dungeon bits of Hour of the Dragon don't hold a candle to this.
The Appendix N entry for Howard doesn't call out particular Conan stories, but I have to imagine that if it did, this one would've made the cut.